The number of Dan Sullivans on Alaska’s ballot for U.S. Senate has been reduced to one.

Dan J. Sullivan, the Republican Senate candidate bearing the same first and last name as Senator Dan S. Sullivan, the incumbent Republican, was ruled ineligible to run for office on Monday by the state’s director of elections.

Republicans in the state had accused Democrats of plotting to confuse voters by inserting Dan J. Sullivan, a former teacher, into the race, which could have taken votes away from the incumbent senator and boosted Mary Peltola, the Democratic candidate seeking to oust him. Both Ms. Peltola and the challenger named Sullivan have denied any coordination, and he has insisted that his bid was a good-faith effort.

Republican state officials responsible for overseeing elections disagreed. Lt. Gov Nancy Dahlstrom began investigating his candidacy last week, and Carol Beecher, the state elections director, said in a preliminary ruling last Wednesday that she was likely to kick him off the ballot unless he could offer compelling evidence of his legitimacy as a candidate.

On Monday, Ms. Beecher wrote that she had not received any such evidence.

“A declaration of candidacy filed for the purpose of confusing or misleading voters and compromising the fairness of the ballot is not properly filed,” Ms. Beecher wrote. “I am de-certifying your candidacy for United States Senator.”

Ms. Beecher cited several pieces of evidence for her decision. She wrote that his attempt to be listed on the ballot as “Dan Sullivan” rather than his registered name of “Daniel J. Sullivan, Jr.” appeared to be an effort to confuse voters about which Sullivan he was.

“Indeed, you yourself appeared to be confused when you initially emailed the Division asking to be listed on the ballot as ‘Dan S. Sullivan.’ ‘S’ is Senator Sullivan’s middle initial, not yours,” Ms. Beecher wrote.

She also noted that he had registered as a Republican just days before filing to run for office, and that he has been working with a longtime Democratic consultant. She even cited as evidence the fact that his campaign website’s theme and color scheme were similar to the senator’s.

Ms. Beecher wrote that Mr. Sullivan had 30 days to appeal the ruling, but ballots were scheduled to be printed on June 28, so it was unclear if he needed to win an appeal before that date in order to appear on the ballot.

Republicans quickly celebrated the ruling. “Alaskans can choose their next senator without a sham candidate whose primary purpose was to confuse Alaskan voters, treat Alaskans with contempt, and rig the election for Peltola,” the incumbent senator’s campaign said in a statement.

Neither Ms. Peltola’s campaign nor Dan J. Sullivan immediately responded to requests for comment. Last week, Mr. Sullivan said in a statement that he was “a qualified candidate who followed the rules and filed to run for office under my legal name. Yet, unsupported accusations have been given credibility while political operatives continue their effort to keep me off the ballot.”